A key insight was provided by a series of papers showing the biases introduced by fitting
with a single isothermal model complex spectra with multi-temperature components.
In the spectra extracted from large apertures in the bright cores of galaxy groups and ellipticals temperature and abundance gradients are present [1]}, [2]}, [3]}, [4]}. The very sub-Solar abundances obtained from previous studies were an artefact of fitting isothermal models and two-temperature models provided better fits to the data and higher metallicities.
This is the Fe-bias also described in the previous section and demonstrated by means of simulations of ASCA spectra [4]}. The discovery of the Fe bias highlighted the importance of the ability of performing spatially resolved spectroscopy and the difficulties in the modelization of the thermal and abundance structure in the cores of galaxy, groups and clusters, as it was shown at those times by the early results of M87 with XMM-Newton [6]}.
